1. Field of the Invention
The present invention relates to a steering control apparatus for a vehicle and, more particularly, to a steering control apparatus having an electrically operated power steering apparatus.
2. Description of the Related Art
A conventional steering control apparatus, which is provided with a power steering mechanism for assisting steering effort, comprises a steering apparatus transmitting a motion of a steering wheel to a rotary valve via an input shaft, a torsion bar and an output shaft. The rotary valve selectively directs a hydraulic pressure generated by an oil pump to power cylinders so as to reduce a steering effort applied to the steering wheel.
Such a conventional steering apparatus includes a valve control mechanism which reduces rigidity of steering gears such as a torsion bar. This causes a problem that a rigid feeling at or near a neutral position of the steering wheel is reduced. In order to solve this problem, Japanese Utility Model Publication No. 2-45109 suggests a steering apparatus which provides a preset load to a torsion bar at a neutral position by using a spring. The steering apparatus disclosed in the above-mentioned patent publication is of a rotary valve type, and has an input shaft formed with a V-shaped groove. A portion of an output shaft surrounding the input shaft has a through opening aligning with the V-shaped groove formed on the input shaft. A ball is provided in the through opening of the output shaft. The ball is urged to the V-shaped groove by an annular spring so that a preset pressure is provided to a torsion bar at a neutral position.
The above-mentioned conventional steering apparatus can provide a rigid feeling at the neutral position by providing a preset pressure to the torsion bar at the neutral position. Thus, straight line stability of a vehicle is improved.
However, in the above-mentioned conventional steering apparatus for a vehicle, no consideration is given to hysteresis between a steering torque and a valve angle formed between a valve housing and a rotary valve. Thus, a magnitude of such hysteresis cannot be controlled to an optimum level.
Recently, a steering apparatus has been suggested which uses an electrically operated power assisting mechanism having a power unit such as an electric motor instead of the above-mentioned hydraulically operated power assisting mechanism. However, the above-mentioned hysteresis is also not considered for the steering apparatus having the electrically operated power steering mechanism. Thus, neither the steering apparatus having the electrically operated power steering mechanism nor the steering apparatus having the hydraulically operated power steering mechanism can provide an optimum steering characteristic.